mq_open(3C)

Name

Synopsis

Description

The mq_open() function establishes the connection between a process and a message queue with a message queue descriptor. It creates a open message queue description that refers to the message queue, and a message queue descriptor that refers to that open message queue
description. The message queue descriptor is used by other functions to refer to that message queue.

The name argument points to a string naming a message queue. The name argument must conform to the construction rules for a path-name. If name is not the name of an existing message queue and its creation is not requested,
mq_open() fails and returns an error. The first character of name must be a slash (/) character and the remaining characters of name cannot include any slash characters. For maximum portability, name should
include no more than 14 characters, but this limit is not enforced.

The oflag argument requests the desired receive and/or send access to the message queue. The requested access permission to receive messages or send messages is granted if the calling process would be granted read or write access, respectively, to a file with the equivalent
permissions.

The value of oflag is the bitwise inclusive OR of values from the following list. Applications must specify exactly one of the first three values (access modes) below in the value of oflag:

O_RDONLY

Open the message queue for receiving messages. The process can use the returned message queue descriptor with mq_receive(3C), but not mq_send(3C). A message queue may be open multiple times in the same or different processes for receiving messages.

O_WRONLY

Open the queue for sending messages. The process can use the returned message queue descriptor with mq_send(3C) but not mq_receive(3C). A message queue may be open multiple times in the same or different processes for sending messages.

O_RDWR

Open the queue for both receiving and sending messages. The process can use any of the functions allowed for O_RDONLY and O_WRONLY. A message queue may be open multiple times in the same or different processes for sending messages.

Any combination of the remaining flags may additionally be specified in the value of oflag:

O_CREAT

This option is used to create a message queue, and it requires two additional arguments: mode, which is of type mode_t, and attr, which is pointer to a mq_attr structure. If the pathname, name
, has already been used to create a message queue that still exists, then this flag has no effect, except as noted under O_EXCL (see below). Otherwise, a message queue is created without any messages in it.

The user ID of the message queue is set to the effective user ID of process, and the group ID of the message queue is set to the effective group ID of the process. The file permission bits are set to the value of mode, and modified by clearing all bits set in the file
mode creation mask of the process (see umask(2)).

If attr is non-NULL and the calling process has the appropriate privilege on name, the message queue mq_maxmsg and mq_msgsize attributes are set to the values of the
corresponding members in the mq_attr structure referred to by attr. If attr is non-NULL, but the calling process does not have the appropriate privilege on name, the mq_open
() function fails and returns an error without creating the message queue.

O_EXCL

If both O_EXCL and O_CREAT are set, mq_open() will fail if the message queue name exists. The check for the existence of the message queue and the creation of the message queue if it does not exist
are atomic with respect to other processes executing mq_open() naming the same name with both O_EXCL and O_CREAT set. If O_EXCL and O_CREAT are not set, the result is undefined.

O_NONBLOCK

The setting of this flag is associated with the open message queue description and determines whether a mq_send(3C) or mq_receive(3C) waits for resources or messages that are not currently available, or fails with errno set to EAGAIN. See mq_send(3C) and mq_receive(3C) for
details.